On the simulation of Fraunhofer radiation patterns in the Fresnel region

Physical limitations on the size of obstacle-free test sites give rise to the need of making radiation-pattern measurements on high-gain antennas at a reduced distance. The general practice is to defocus the primary source along the principal axis of the antenna reflector by a small distance so that Fraunhofer patterns may be simulated in the Fresnel region. This note summarizes and compares three different approaches with which the proper amount of defocus may be determined.